1. Field of the Invention
The present invention relates to a solid polymer electrolyte type fuel cell and in particular to a solid polymer electrolyte type hydrogen-oxygen fuel cell.
2Description of Related Art
The present invention can be applied to generic solid polymer electrolyte type fuel cells. The solid poller electrolyte type fuel cells generally comprise two current collectors, a solid polymer electrolyte membrane (hereinafter referred to as "electrolyte membrane"), two electrodes between which said electrolyte membrane is interposed, and means for feeding hydrogen and oxygen as fuels. The electrodes comprise a catalyst, a carrier for supporting the catalyst, an ion (proton) conductor of the same solid polisher electrolyte as the above electrolyte and a binder for binding them. The two electrodes are a hydrogen electrode and an oxygen electrode and the electrochemical reactions in these electrodes are as follows.
In the hydrogen electrode, a hydrogen molecule is ionized into protons to release electrons. This can be shown by the following formula (1). EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- ( 1)
The protons transfer through the ion conductor in the electrode and reach the electrolyte and further pass through the electrolyte and transfer to the oxygen electrode on the opposite side. On the other hand, the electrons released transfer to the oxygen electrode through the external circuit. In the oxygen electrode, the protons combine with the electrons released from the hydrogen electrode to form water in accordance with the following formula (2). EQU 1/2O.sub.2 +2H.sub.2 +2e.sup.- .fwdarw.H.sub.2 O (2)
The above reaction process of fuel cells comprises mainly the following four stages.
(A) Diffusion of hydrogen and oxygen onto the surface of the catalyst. PA1 (B) Reactions on the surface of the catalyst in the hydrogen electrode and the oxygen electrode. PA1 (C) Conduction of protons in both the electrodes and in the electrolyte. PA1 (D) Discharging of water.
The degree of diffusion of the fuel gases and the degree of reaction rate of the fuel gases in the respective stages greatly affect the output of the cell.
In the stage (A), it has been proposed for efficient feeding and diffusion of the fuel onto the surface of the catalyst to use a corrugated current collector as shown in FIG. 1 of Japanese Patent Application Kokai (Laid-Open) No. 60-35472 and a carbon plate having rectangular grooves as disclosed in Japanese Patent Application Kokai (Laid-Open) Nos. 3-102774 and 2-86071. When said corrugated current collector or the grooved side of said carbon plate having rectangular grooves is allowed to contact with the electrode, spaces are formed therebetween and the fuel diffuses through these spaces onto the surface of the electrode.
In the solid polymer electrolyte type fuel cells, the above-mentioned structure is generally employed and the output is developed to some extent.
The protons which have passed through the electrolyte membrane produce water at the interface between the electrolyte membrane and the oxygen electrode because the reaction of the above formula (2) proceeds at the interface, and especially at a high current density, a water film is formed and so-called flooding phenomenon occurs. This water film causes decrease in contacting efficiency between the catalyst and the oxygen gas which has diffused through the oxygen electrode. As a result, reduction of output density is apt to occur and the cell performance becomes unstable. This flooding phenomenon tends to occur especially at the interface between the oxygen electrode and the electrolyte membrane. Thus, it is necessary to eliminate the produced water from the system.
U.S. Pat. No. 4,643,957 proposes to use a water repellant in the electrodes of phosphoric acid type fuel cells.